1. Field of the Invention
The present invention relates to a display panel used in, for example, the display devices for instrument gauges equipped in a vehicle or the like, and a method for producing the display panel.
2. Description of the Related Art
Conventionally, vehicles such as automobiles for example, are provided with an arrangement of display devices for instrument gauges in the dashboard facing the driver's seat. In general, as set forth in Japanese Patent Laid-open (unexamined) No. 2001-343260, a display device for vehicles (automotive instrument panel) includes a display panel having graphical design parts (printed layer) containing scales, characters and the like, and a light source disposed on the rear side of this display panel. In the display panel, the parts excluding the scales or characters of the graphical design parts are formed as opaque parts which do not allow light transmission, while the parts having scales or characters are formed as transmissive parts which allow light transmission. On the basis of such configuration, when the display panel is illuminated with the light source during night time, the transmissive parts such as the characters, scales and the like of the display panel can be brightly displayed. Furthermore, since the display panels used in the display devices for vehicles are demanded to have a high quality feel, visibility, texture and the like, the recent trend is directed to the use of display panels having three-dimensional shapes, produced by heat molding such as vacuum molding, air pressure molding or insert molding.
Such a display panel is usually produced by printing opaque parts (solid concealed image parts) on the surface of a transparent substrate made of a resin such as polycarbonate, using a screen printing method. After the printing process, the display panel may be subjected to cutting processing so as to make the external shape into a desired shape, or may be subjected to punching processing in order to form holes or the like on the display panel.
The screen printing method is a method of producing a screen (plate) on which a print image is drawn from the print data, and performing printing on a substrate through this screen using, for example, a solvent drying type ink, a thermocurable type ink or the like. This printing method is advantageous in that printing can be done at one time such that the print density of the opaque parts would be high. However, since the screen printing method is carried out by way of monochromatic printing, multilayer printing should be performed using inks of different colors in order to form graphical designs such as characters. Thus, there is a problem that the number of processing steps or the processing time may be increased. Furthermore, the screen printing method has a problem that there is limitation in the applicable designs, because accuracy of the printing position or resolution is generally low.
For the display panel for vehicles such as passenger cars, a display panel having a variety of graphical design parts formed in accordance with the vehicle model, engine displacement, vehicle grade and the like, is demanded. That is, the print contents to be formed on the display panel would vary with the type of the vehicle. Since it is required in the screen printing method to go through the procedure of replacing the plate and inks, setting the printing conditions and the like, whenever the type of the display panel is changed, there is a problem of an increase in the costs.
In addition, the screen printing method, which is a mass printing technique, is not suitable for manufacturing a small quantity of products such as trial products or supply products. In other words, there is a problem that processes such as plate making and lithographic plate making, which are incidental to the method, add to an increase in the costs.
On the other hand, in recent years, a remarkable progress has been achieved in the development of a printing method called on-demand printing, in which direct image drawing is possible, without the need to produce a plate from the print data. In particular, an inkjet method is a method of performing printing by spraying ink from electronically controlled printhead nozzles. Since this method requires a simple mechanism of apparatus and low initial costs, and exhibits high image resolution, the method is being rapidly adopted in the field of printers for office use and the like. Furthermore, long life type printers using solvent-based inks have also been developed for the use in signboards and the like.
However, employing the inkjet method in the formation of a printed layer of the display panel faced the following problems.
Specifically, because the above-described display panel used in vehicles and the like require optical transparency and heat resistance, a material such as polycarbonate or polyethylene terephthalate needs to be used as the base material of the display panel. However, when inkjet printing is performed on such material using an aqueous ink, ink droplets spatter on the surface, while in the case of a solvent-based ink, there is a risk that ink droplets aggregate before they dry, or the ink droplets dissolve the substrate, thus resulting in substrate deformation.
In recent years, as set forth in Japanese Patent Laid-open (unexamined) No. 2003-261,799 and No. 2006-8,998, inkjet printing methods using a UV-curable ink have been developed, and as set forth in Japanese Patent Laid-open (unexamined) No. 2006-214,906, a technology of producing display panels by forming a printed layer on a resin substrate using a UV-curable ink has been developed.
With regard to the inkjet printing method using UV-curable ink, it is required, in order to maintain the image quality above a certain level, that the UV-curable ink be cured within several seconds after the UV-curable ink jetted from nozzles of the printhead of an inkjet printing apparatus impacts on a substrate. Therefore, an inkjet printing apparatus employs a system in which a printhead and a UV irradiator are disposed side by side so that exposure can be carried out immediately after jetted ink impacts on a substrate. Meanwhile, the UV-curable ink is also required to have a tendency to be easily cured by UV irradiation and to be highly reactive.
The display panel used in the display devices for vehicles is often subjected to molding by means of a mold having a predetermined shape as described above, or subjected to punching processing, cutting processing or the like. For this reason, it has been desirable to use an ink having excellent stretchability and adhesiveness, as the UV-curable ink.
If a display panel is produced using an ink having insufficient stretchability and adhesiveness, there is a risk of cracks occurring in the printed layer at the time of subjecting the display panel to molding by means of a mold having a predetermined shape, or to punching processing, cutting processing or the like.
It has also been desirable to use an ink which would not cause tackiness at the surface of the printed layer even after the ink has been cured, in the display panel.
For that reason, it has been desirable for the UV-curable ink to have curability to a certain extent.
However, the UV-curable inks that are currently under development, are inks having excellent stretchability and adhesiveness but insufficient curability, or inks having excellent curability but insufficient stretchability and adhesiveness.
Since the combination of stretchability and adhesiveness, and curability are generally properties conflicting with each other, it has been very difficult to develop an ink having the properties of both sides to very high levels.
In particular, when the display panel is used in, for example, a display device for vehicles as described above, the display panel is disposed within a casing. For this purpose, inserting a part of the display panel into the casing has been tried so as to fix the display panel within the casing. In this case, the display panel is pressed at the fitting part of the casing and the display panel, and the pressed part may be subject to a stress as large as, for example, 1 MPa. Thus, in this case, it is desirable to use a UV-curable ink having particularly high curability, so that generation of cracks and the like can be avoided in the printed layer of the display panel. However, if an ink having curability so high to the extent of preventing cracks upon pressing is used, the stretchability and adhesiveness are impaired. Therefore, in this case, there is a problem that molding and processing of the display panel become difficult.
Accordingly, there has been a need to determine the design, in the case of producing a display panel through the technique of inkjet printing using a UV-curable ink, while taking into consideration the use of the display panel, the position of the pressed part, and the subsequent processes such as molding and processing. In other words, there has been a need to reduce the amount of molding or the amount of processing, or to design the display panel not to have a printed layer provided at the pressed part, thereby limitation being imposed on the design of display panels.
On the other hand, the UV-curable inks that are currently under development have excellent stretchability and adhesiveness, but on the other hand, have a problem of insufficient curability. For that reason, with regard to the display panel produced using such a UV-curable ink, there is a risk that tackiness may still remain at the surface of the printed layer even after the ink has been cured, and have deteriorated durability of the printed layer. Specifically, there is a risk that, for example, the ink may leave marks when there occurs a contact with the printed layer after curing, or that if a display panel having the printed layer formed therein is stored in piles, the display panels may adhere to each other through the printed layer, or the printed layer may be transferred to the display panel.
Meanwhile, UV-curable inks having excellent curability are also under development. However, the inks having excellent curability have insufficient stretchability and adhesiveness. For that reason, in the display panels produced using such UV-curable inks, there is a risk that cracks may be generated in the printed layer when the display panel is subjected to molding by means of a mold having a predetermined shape, or subjected to punching processing, cutting processing or the like. Also, at the time of applying a casing, there is a risk that cracks may be generated in the printed layer because of the stress exerted on the fitting part of the display panel and the casing of the display device or the like.